EDIT: I understand that it’s supposed to be for safety. I’m asking because I’ve only experienced the stopping mechanism when I pull. I’ve hit the brakes very abruptly before but nothing. Should I get it looked at?
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Because that is literally their function – to stop you from suddenly lurching forward and pulverizing your skull against the windshield.
Because if they didn’t they wouldn’t be very useful restraining people in a crash.
If you mean to ask how they do it, there is a mechanism that spins as the belt extends, flinging out an arm that catches on a toothed housing. This locks it in place as long as tension is applied.
Such a mechanism allows you to pull the belt out to put it on, but also stops you from going through the windshield in a collision.
Imagine you get into a car crash. Your car stops where it is, but your body keeps moving until it hits something.
If you weren’t wearing your seatbelt, or its retention mechanism failed, then you’d fly forward and crush yourself against your steering wheel or the passenger dashboard or against the back of the seat in front of you.
Instead, the seatbelt is designed so that if it experiences such a rapid motion, it locks itself so that you *don’t* fly forward.
Safety? Did you think it was a fashion statement?
Try that again (carefully) while you are already leaning forward a bit. Braking abruptly may not be enough to do the trick if you’re leaning back – there may not be enough momentum left to trigger the seatbelts if your torso coming out of your seat absorbs most of it.
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That’s a feature, not a bug. The kind of force that would cause the belt to stop, is the kind when involved in a collision. The belt stopping due to that high of a force is what helps keep the occupant restrained.
> I’ve hit the brakes very abruptly before but nothing.
Were you at risk of flying out the windscreen in that situation? No, which is why it didn’t go off.
They stop when you pull them hard because the mechanism that makes the seatbelt lock up during a crash is (usually) a simple one that can’t differentiate. Basically, inside the mechanism there’s a U-shaped or J-shaped channel that belt runs through with a ball or roller inside, and the channel gets narrower near the top end. Normally, gravity keeps the ball near the bottom of the channel, which has lots of room, so the seatbelt slides right by when you pull it. But during a hard braking situation, the momentum forces the ball upwards into the narrow part of the channel, and the seatbelt sliding forward at that same time will lock it into place. However, because of the simplicity, if you pull the belt quickly, the ball won’t have time to roll, and will instead get pulled upwards into the channel, causing it to lock. This is also why you get issues in older cars, because those channels can get dirty over time.
I remember having read in some children book that in some modifications the seatbelts are held by one time pyro devices which explode and fixate the belt in case of emergency. Boy was I afraid to pull them roughly!
Was such a design ever implemented in older cars or was the author joking?
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